Photocatalytic CO2Reduction Using a Robust Multifunctional Iridium Complex toward the Selective Formation of Formic Acid

Article abstract of DOI:10.1021/jacs.0c03097

A highly efficient tetradentate PNNP-type Ir photocatalyst, Mes-IrPCY2, was developed for the reduction of carbon dioxide. The photocatalyst furnished formic acid (HCO2H) with 87percent selectivity together with carbon monoxide to achieve a turnover number of 2560, which is the highest among CO2 reduction photocatalysts without an additional photosensitizer. Mes-IrPCY2 exhibited outstanding photocatalytic CO2 reduction activity in the presence of the sacrificial electron source 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) in CO2-saturated N,N-dimethylacetamide under irradiation with visible light. The quantum yield was determined to be 49percent for the generation of HCO2H and CO. Electron paramagnetic resonance and UV-vis spectroscopy studies of Mes-IrPCY2 with a sacrificial electron donor revealed that the one-electron-reduced species is the key intermediate for the selective formation of HCO2H.

Full text of DOI:10.1021/jacs.0c03097

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Communication
2
Photocatalytic CO Reduction Using a Robust Multifunctional
Iridium Complex towards the Selective Formation of Formic Acid
Kenji Kamada, Jieun Jung, Wakabayashi Taku, Keita Sekizawa, Shunsuke
Sato, Takeshi Morikawa, Shunichi Fukuzumi, and Susumu Saito
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c03097 • Publication Date (Web): 26 May 2020
Downloaded from pubs.acs.org on May 26, 2020
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Photocatalytic CO2 Reduction Using a Robust Multifunctional Irid-
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Kenji Kamada,† Jieun Jung*,† Taku Wakabayashi,† Keita Sekizawa,‡ Shunsuke Sato,‡ Takeshi Morikawa,‡
,†,||
§
Shunichi Fukuzumi, and Susumu Saito*
†Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
‡Toyota Central R&D Labs., Inc., 41-1 Yokomichi, Nagakute 480-1192, Japan
§Faculty of Science and Engineering, Meijo University, Nagoya 468-8502, Japan
||Research Center for Materials Science (RCMS), Nagoya University, Chikusa, Nagoya 464-8602, Japan.
Supporting Information Placeholder
Several examples of single-active-site photocatalysts that function
ABSTRACT: A highly efficient tetradentate PNNP-type Ir photo-
catalyst, Mes-IrPCY2, was developed for the reduction of carbon di-
oxide (CO2). The photocatalyst furnished formic acid (HCO2H)
with 87% selectivity together with carbon monoxide (CO) to
achieve a turnover number of 2560, which is the highest among CO2-
reduction photocatalysts without an additional photosensitizer.
Mes-IrPCY2 exhibited outstanding photocatalytic CO2-reduction
activity in the presence of the sacrificial electron source 1,3-dime-
thyl-2-phenyl-2,3-dihydro-1H-benzo[d]-imidazole (BIH) in CO2-
saturated N,N-dimethylacetamide (DMA) under irradiation with
visible light. The quantum yield was determined to be 49% for the
generation of HCO2H and CO. Electron paramagnetic resonance
(EPR) and UV-vis spectroscopy studies of Mes-IrPCY2 with a sac-
rificial electron donor revealed that the one-electron reduced species
is the key intermediate for the selective formation of HCO2H.
as both the photosensitizer and catalyst for CO2 reduction based on
Re,7 Os,8 Ir,9 Ru,10 and other metals11 have been reported. The devel-
opment of self-photosensitized metal complexes is advantageous in
terms of lowering the activation energy of the catalytic reaction and
controlling its selectivity; thus, extensive efforts have been devoted
to designing new molecular photocatalysts by changing the metal
center and/or ligands. However, the development of robust homo-
geneous photocatalysts has remained challenging as most exhibit
low turnover numbers (TON), and photocatalysts that can produce
HCO2H selectively by CO2 photoreduction are very rare.7d,9e
Herein, we introduce Mes-IrPCY2 (1) as a structurally robust,
tetradentate PNNP-type Ir complex (Figure 1, inset) for the photo-
catalytic reduction of CO2 to HCO2H. This CO2-reduction photo-
catalyst produces mainly HCO2H with high activity and selectivity
without requiring an additional photosensitizer. From a molecular
engineering perspective, the key aspects for the design of this catalyst
are: i) the introduction of a bulky PNNP-ligand, which has been
The development of systems for the synthesis of value-added or-
ganic substances from carbon dioxide (CO2) has become increas-
ingly popular as a key strategy to solve the problems of global warm-
ing and fossil fuel shortages.1,2 In particular, formic acid (HCO2H),
a platform chemical that can be obtained from the reduction of CO2
and used in applications such as direct formic acid fuel cells
(DFAFCs),3 could represent a valuable energy-storage source. Pho-
tocatalytic CO2 reduction has attracted extensive interest, since the
photocatalytic conversion of CO2 to energy-enriched compounds
could potentially be achieved under relatively mild conditions.4
However, because the homogeneous photochemical reduction of
CO2 is an inherently difficult-to-control multi-electron reduction,
two-component systems involving a transition-metal catalyst, pho-
tosensitizer, and sacrificial reductant or supramolecular systems
have typically been used to achieve the photocatalytic reduction of
CO2.5,6
2500
+
R
R
R = Mes
2000
1500
1000
500
0
H
Ir
N
N
P
P
Cy2
Cy2
Cl
1, Mes-IrPCY2
Scheme 1. Photocatalytic Reduction of CO2 with an Ir Complex
under Photoirradiation (λ ≥ 400 nm)
0
50
100
150
Time, h
Figure 1. Time course plots of the products [HCO2H (red), CO (blue),
and H2 (black)] obtained during the photocatalytic reduction of CO2 with
a catalytic amount of Mes-IrPCY2 (20 µM) and BIH (0.2 M) in a CO2-sat-
urated mixture of DMA:H2O (9:1, v/v) under photoirradiation (λ ≥ 400
nm) at 298 K. The inset shows the chemical structure of Mes-IrPCY2 (1).
hν (λ ≥ 400 nm)
Mes-IrPCY2
CO2
HCO2H + CO + H2
electron donor
DMA / H2O (v/v = 9:1)
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shown to prevent catalyst deterioration and promote efficient hy-
Page 2 of 13
drogenation,12 and ii) the incorporation of bipyridyl CH2P groups,
which could potentially act as proton donors. The bulky PNNP lig-
ands were expected to control the stereochemistry at the metal atom
and effectively reduce CO2 under photoirradiation conditions.13
Given that metal complexes that bear bipyridyl CH2P groups can act
as hydrogenation catalysts via an outer-sphere mechanism, we antic-
ipated that 1 would successfully produce HCO2H by CO2 photore-
duction via outer-sphere catalysis, accompanied by inner-sphere ca-
talysis to produce CO, as many previous examples have suggested.7,
8b, 9c, 10a, 11 In other words, 1 is potentially a multifunctional photocata-
lyst that could function as both a photosensitizer and a catalyst that
could reduce CO2 to HCO2H and CO via outer-sphere and inner-
sphere catalysis, respectively.
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(a)
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0.02
0.01
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650
450 500 550 600
Wavelength, nm
700 750
The photocatalytic reduction of CO2 was examined by photoirra-
diation (λ ≥ 400 nm) of a mixed dimethylacetamide (DMA) and
H2O (v/v = 9:1) solution containing 1 and the sacrificial electron
donor 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole
(BIH) under 1 atm of CO2 (Scheme 1). Efficient CO2 reduction oc-
curred, with TONs of 2080(50), 470(10), and 15(1) for HCO2H,
CO, and H2, respectively. Plots of the time course of the formation
of each product by 1 are shown in Figure 1; the amount of HCO2H
and CO produced continually increased with irradiation time for
over 1 week, indicating that 1 exhibits sufficient robustness in this
photocatalytic reaction.14 A mercury test also revealed that photo-
catalytic reduction of CO2 occurred by homogeneous catalyst (Ta-
ble S1; entry 1 and 2), exhibiting no significant difference in the
amount of product in the presence of Hg (0.17 M). Negligible
amounts of the products were produced in control experiments in
the absence of 1 or CO2 (Figure S1). A labeling experiment was per-
formed with 13C-labeled CO2 (13CO2) in a 13CO2-saturated mixture
of DMF-d7/ H2O (v/v = 9:1) to determine the carbon source of the
products. The 13CO2-labeling experiments indicated that the CO2
gas was the source of the carbon atoms in the generated HCO2H
(Figure S2). The quantum yield (QY = 49% at λ = 400 nm) was de-
termined using a ferroxalate actinometer,15 and the selectivity to-
wards HCO2H was 87% (cf. experimental section and Figure S3).
When triethanolamine (TEOA), which is commonly used as an elec-
tron donor in this field, was used instead of BIH, however, the reac-
tivity decreased significantly and the selectivity for HCO2H was lost
(Figure S4 in SI).
(b)
12
10
8
6
4
0
1.5
[Ferrocene], mM
Figure 2. (a) Transient absorption spectral changes (red: 2 ns; orange: 20
ns; green: 50 ns; blue: 200 ns; black: 500 ns) after sub-nanosecond laser
excitation at 355 nm in a deaerated DMA solution of 1 (1.0 mM) at 298 K.
Inset shows the decay time profile of the absorbance at 500 nm due to the
decay of the excited state of 1. (b) Plot of kobs vs the concentration of ferro-
cene in a DMA solution at 298 K.
2.0
2.5
0.5
1.0
9.5
6
5
9.0
4
3
8.5
8.0
In order to study the photophysical properties of 1, sub-nanosec-
ond laser-induced transient absorption (picoTAS) measurements16
were performed on 1. Laser excitation of a deaerated DMA solution
of 1 resulted in the formation of a long-lived excited state with an
absorption band at λmax = 500 nm (Figure 2a). This new absorption
was assigned to the triplet (T1) excited state. Intersystem crossing
processes from the singlet (S1) excited state to the T1 state are known
to be extremely rapid due to strong spin-orbit coupling.17 From the
decay time profile of the absorbance at 500 nm, the lifetime of the T1
2
7.5
1
7.0
0
0.5
1.0
1.5
Eox, V vs SCE
Figure 3. Plot of log ket of photoinduced electron transfer from
methoxybenzene derivatives and ferrocene derivatives [1: 1,4-
dimethoxybenzene; 2: 1,2,3,4-tetramethoxybenzene; 3: 1,2,4-
trimethoxybenzene; 4: triphenylamine; 5: bromoferrocene; 6: ferrocene]
to the excited state of 1 in DMF at 298 K.
state of 1 was determined to be t = 173(13) ns at 298 K (Figure 2a,
inset).
and methoxybenzene derivatives to the T1 excited state of 1 were de-
termined in the same manner, and the ket values are listed in Table
S2 (Figures S5–S9). The plot of log ket as a function of the one-elec
tron oxidation potentials of the electron donors (Eox) (Figure 3) ex-
hibits the expected behavior, i.e., the log ket value increases with de-
creasing Eox to reach a diffusion-limited maximum, as expressed by
the Marcus equation of intermolecular electron transfer (eq 1):
When ferrocene was added to a deaerated DMA solution of 1 as
an electron donor, the decay of the absorbance at 500 nm due to the
T1 excited state of 1 was accelerated, and the decay rate constant in-
creased linearly with increasing concentration of ferrocene (Figure
2b). These results indicate that electron transfer occurred from fer-
rocene to the T1 excited state of 1. The rate constant of electron
transfer from ferrocene to the T1 excited state of 1 was determined
to be [(3.2 0.3) × 109 M−1 s−1 at 298 K] from the slope of the linear
plot of kobs as a function of the ferrocene concentration (Figure 2b).
The rate constants of electron transfer (ket) from various ferrocene
1/ket = 1/kdiff + 1/(Zexp[(–λ/4)(1 + ∆Get/λ)2/(kBT)])
(1)
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Scheme 2. Proposed Mechanism of the Photocatalytic Reduction of CO2 Using Mes-IrPCY2 (1)
1
2
H
O
O
H
C
H
Mes
CO2
3
4
5
6
7
8
9
10
11
12
13
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17
P
N
BI•, H+
•–
Mes
Ir
CO , H2O, BI+
H+, BIH
hν
N
P
N
P
Mes
BIH
•–
Ir
L
N
hν
OERS
P
Mes
a
H+
L
COOH
H
Mes
+
Mes
Mes
Transition state
+
spontaneous
transfer of
H+ and H–
P
HCO2H
P
P
N
b
N
N
IrI
IrIII
IrIII
N
N
N
∆ or hν
O
H
H
P
C
O
Mes
P
P
Mes
Mes
L
L
L
1
Ir(I)
IrIII–COOH
Mes
Mes
P
N
P
BI+
BI•, 2H+
Mes
•–
N
Ir
IrII
N
N
P
Mes
CO2
P
HCO2H
P = PCy2
L
L
L = Cl– or solvent
Intermediate
1’
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where λ is the reorganization energy of electron transfer, kdiff is the
diffusion rate constant, Z is the collision frequency, which is taken as
1011 M–1 s–1, kB is the Boltzmann constant and T is the absolute tem-
perature.18,19
The Gibbs energy change associated with the electron transfer,
∆Get, is given by eq 2:
by electron paramagnetic resonance (EPR) measurements of a
mixed DMA/H2O (v/v = 9:1) solution containing 1 and BIH under
photoirradiation at 173 K; a new EPR signal corresponding to the
OERS (g1 = 2.01; g2 =2.00; g3 =1.95 and line widths of ∆B1 = 12 G,
∆B2 = 13 G, and ∆B3 = 37 G) is shown in Figure 4.21,22 The electro-
chemical characterization of 1 indicates that after the one-electron
reduction of 1 at –1.22 V vs SCE assignable to a reduction potential
for the ligand of 1 [P(bpy)P/P(bpy•–)P],23 the OERS reacted with
CO2 to give catalytic current growth under CO2 (Figure S14).
When BIH was replaced with TEOA, deprotonation of IrIII-H to
give an IrI species (Scheme 2, reaction pathway b) rather than the
OERS was predominantly observed (Figure S15). The rate constant
for the deprotonation of 1 is independent of the concentration of
TEOA (Figure S15c). 1H NMR spectrum of Ir(III)–H showed dis-
appearance of the 1H NMR triplet signal of the hydride by photoirra-
diation, indicating deprotonation of Ir(III)–H occurred to form
Ir(I) without BIH nor TEOA. Additionally, it was observed that the
hydrogen atoms of CH2P group are all replaced with deuterium at-
oms, indicating that the methylene hydrogen atoms are acidic
enough to be deprotonated under photoirradiation in DMA-d9/D2O
(9:1, v/v) (Figure S16). The deprotonation of the excited state of 1
is also consistent with a previous report24 that revealed that the
metal-to-ligand charge-transfer due to the excitation reduces the ba-
sicity of the Ir center, facilitating the release of a proton from the Ir
complex (Figure S17; DFT calculations).
Based on the experimental results described above, a mechanism
for the photocatalytic reduction of CO2 by 1 in the presence of an
electron donor is proposed in Scheme 2. In reaction pathway a,
photo-excited IrIII–H (1) is reduced to OERS by fast electron trans-
fer from BIH, followed by the insertion of CO2 into the Ir-H bond to
give the corresponding transition state and intermediate.25 A nucle-
ophilic attack on CO2 leads to an H-bonded formate intermediate,
followed by dissociation of HCO2H and regeneration of 1. Sponta-
neous proton and hydride transfer cannot be comprehensively ruled
out at this point.26 In reaction pathway b, deprotonation of 1 leads to
the corresponding IrI species, which reacts with CO2 to give the IrIII-
COOH species, followed by release of CO and regeneration of IrI. In
the proposed mechanism, the rapid one-electron transfer process for
the formation of OERS results in the release of HCO2H via outer-
sphere catalysis, whereas the deprotonation of 1 leads to the evolu-
tion of CO via inner-sphere catalysis.27,28
∆Get = e(Eox – Ered
)
(2)
where e is the elementary charge and Ered the one-electron reduction
potential of the electron acceptor. The best fit in Figure 3 gives an
Ered value of 1.32(1) V for [1]*, along with a λ of 0.90(4) eV, and a
k
diff of 7.0 × 109 M–1 s–1.
The electron-transfer rate constant (k2) of BIH was determined to
be (2.5 0.1) × 109 M−1 s−1 (Figure S10), which is significantly larger
than that of TEOA ((1.2 0.1) × 108 M−1 s−1; Figure S11), being
consistent with the more negative one-electron oxidation potential
of BIH (Eox = 0.21 V vs SCE in DMA) compared to that of TEOA
(Eox = 0.68 V vs SCE in DMA) (Figure S12).20
Formation of the one-electron reduced species (OERS) in the
photo-driven reduction of 1 (Scheme 2, reaction pathway a) was ob-
served via the change in the UV-vis spectrum. Upon photoirradia-
tion, a UV-vis spectral change was observed from 1 to a new absorp-
tion band at 569 nm, which showed that the rate constant of the
OERS formation (kobs) increased with increasing concentration of
BIH (Figure S13). The formation of the OERS was also confirmed
g1 = 2.01
g3 = 1.95
g2 = 2.00
3100 3150 3200 3250 3300 3350 3400
Magnetic field, Gauss
Figure 4. X-band EPR spectrum of 1 obtained after 0 min (blue line) and 10
min (red line for experimental and black for simulated) of photoirradiation
(λ ≥ 400 nm) in an Ar-saturated mixture of DMA/H2O (v/v = 9:1) contain-
ing 1 (1.0 mM) and BIH (0.1 M) at 173 K.
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CO2 Reduction to Formate and Electroanalytical Detection of Myricetin
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In conclusion, a new multifunctional PNNP-type Ir complex pho-
tocatalyst (1) has been developed that functions as both a photosen-
sitizer and a catalyst to reduce CO2 to HCO2H via outer-sphere ca-
talysis and to CO via inner-sphere catalysis. The total TON for the
photocatalytic reduction of CO2 is 2560, with 87% HCO2H selectiv-
ity in the presence of BIH as an electron donor. We have clarified the
reaction pathways using experimental data, which indicates that the
initial step of the catalytic cycle is critical for the selective reduction
of CO2. The reactivity and robustness of the catalyst are significantly
enhanced by the introduction of a PNNP-type ligand. The present
study has thus provided new insights into the development of effi-
cient catalysts for CO2 reduction reactions.
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Ko, C.-C. Photochemical and electrochemical catalytic reduction of CO2
with NHC-containing dicarbonyl rhenium(I) bipyridine complexes. Dal-
ton Trans. 2016, 45, 14524–14529. (d) Hameed, Y.; Berro, P.; Gabidullin,
B.; Richeson, D. An integrated Re(I) photocatalyst/sensitizer that activates
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ASSOCIATED CONTENT
Supporting Information.
Experimental details, Scheme S1, Table S1 – S2 and Figures S1 –
S27. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*jieun@chem.nagoya-u.ac.jp
*saito.susumu@f.mbox.nagoya-u.ac.jp
ORCID
Kenji Kamada: 0000-0002-2532-5448
Jieun Jung: 0000-0002-5310-8643
Taku Wakabayashi: 0000-0002-2083-3590
Keita Sekizawa: 0000-0003-2660-0410
Shunsuke Sato: 0000-0001-8178-7367
Takeshi Morikawa: 0000-0002-4985-0925
Shunichi Fukuzumi: 0000-0002-3559-4107
Susumu Saito: 0000-0003-0749-2020
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank Dr. Kin-ichi Oyama (Nagoya University) for elemental
analysis. This work was supported by Grant-in-Aid for Early-Career
Scientists (no. 18K14241 to J. J.), Scientific Research (B) (no.
19H02713 to S.S.), and Grant-in-Aid (no. 16H02268 to S.F.) from
the Japanese Society for the Promotion of Science (JSPS) and Asahi
Glass Foundation (Step-up-grant to S. S.) and a Grant-in-aid for Sci-
entific Research on Innovative Areas (no. 18H04247 to S.S.) from
the Japanese Ministry of Education, Culture, Sports, Science and
Technology (MEXT), as well as from the Ministry of the Environ-
ment Government of Japan.
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[Os(NN)(CO)2Cl2] Complexes. Chem.–Eur. J. 2011, 17, 4313–4322. (b)
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TOC Graph
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Highly robust
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